272 FISH HATCHERY MANAGEMENT 



possible adverse effects on the pond environment. Some treatment chemi- 

 cals are algicidal or herbicidal and may kill enough plants to ultimately 

 cause an oxygen deficit. Other chemicals, such as formalin, may reduce 

 dissolved oxygen levels as they degrade. 



As in prolonged- bath treatments, it is important that the chemical be 

 evenly distributed throughout the culture unit to prevent the occurrence of 

 hot spots. Special boats are available for applying chemicals to ponds. 

 However, such chemical boats are fairly expensive and are not needed un- 

 less large acreages are involved. For dry chemicals that dissolve rapidly in 

 water, such as copper sulfate or potassium permanganate, burlap or any 

 coarse-weave bags can be used. The required amount of chemical is put 

 into a bag and towed behind the boat so that the chemical dissolves in the 

 wake of the boat. Liquids and wettable powders can be applied evenly with 

 hand or power sprayers or can be siphoned over the edge of a boat into the 

 prop wash. 



As with the prolonged- bath method, there is no one correct way to apply 

 a chemical evenly to the unit of water to be treated. Rather, the applica- 

 tion will depend on the kind of chemical being used, the equipment avail- 

 able, and the type of unit to be treated. 



FLUSH TREATMENT 



Flush treatments are simple, and consist of adding a solution of the treat- 

 ment chemical at the upper end of a holding unit and allowing it to flush 

 through. It has been used widely at trout and salmon hatcheries, but is sel- 

 dom used at warmwater hatcheries. It is applicable only with raceways, 

 tanks, troughs, or incubators for which an adequate flow of water is avail- 

 able, so that the chemical is completely flushed through the unit or system 

 within a predetermined time. Highly toxic chemicals should be avoided be- 

 cause there is no way to assure a uniform concentration within the unit be- 

 ing treated. 



CONSTANT-FLOW TREATMENT 



Constant-flow treatments are useful in raceways, tanks, or troughs in situa- 

 tions where it is impractical or impossible to shut off the inflowing water 

 long enough to use prolonged baths (Appendix G). 



The volume of water flowing into the unit must be determined accurately 

 and a stock solution of the chemical metered into the inflowing water to 

 obtain the desired concentration. Before the metering device or constant- 

 flow siphon that delivers the chemical is started, enough chemical should 

 have been added to the water in the device to give the desired concentra- 

 tion. Upon completion of the desired treatment period, the inflow of chemi- 

 cal is stopped and the unit is flushed by allowing the water flow to continue. 



